Coupling assembly for coupling a rod to a bone anchoring element, and polyaxial bone anchoring device

ABSTRACT

A coupling assembly for coupling a rod to a bone anchoring element includes a receiving part having a first end, a second end, a recess having a bottom for receiving the rod, and an accommodation space having an opening at the second end of the receiving part for accommodating a head of the bone anchoring element, and a retainer element configured to be inserted into the receiving part from the first end and to hold at least part of the head, the retainer element having a first portion and a spring portion compressible in an axial direction attached to the first portion. When the retainer element is in the accommodation space in a first position, the spring portion extends in the axial direction from the first portion of the retainer element to an axial position between the first end of the receiving part and the bottom of the recess.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/596,170, filed Jan. 13, 2015, which claims the benefit of U.S.Provisional Patent Application Ser. No. 61/926,691, filed Jan. 13, 2014,the contents of which are hereby incorporated by reference in theirentirety, and claims priority from European Patent Application EP14151004.0, filed Jan. 13, 2014, the contents of which are herebyincorporated by reference in their entirety.

BACKGROUND Field

The invention relates to a coupling assembly for coupling a rod to abone anchoring element, and to a polyaxial bone anchoring device withsuch a coupling assembly. The coupling assembly includes a receivingpart with a channel for receiving the rod, an accommodation space foraccommodating a head of a bone anchoring element, and a retainer elementfor retaining the head of the bone anchoring element in the receivingpart. The retainer element has a structure that includes a springportion that is biased in such a manner that the retainer element snapsautomatically onto a head of the bone anchoring element when the head isbeing inserted.

Description of Related Art

From US 2013/0150852 A1, a polyaxial bone anchor including a housing, abone screw, and a retainer for pivotably coupling the head of the bonescrew to the housing is known. The retainer is positioned into the boreof the housing and includes a plurality of alternating tabs and slotscircumferentially arranged to define a cavity for receiving the headportion of the bone screw therein. The bone anchor further includes aresilient spring means biasing the retainer towards the lower end of thehousing. The head portion of the bone screw may apply a force againstthe retainer opposing and overcoming the biasing force of the resilientspring means. The resilient spring means may be, for example, a wavewasher, a helical spring, an elastomeric member, etc. or may becircumferential or helical slots formed in the retainer.

U.S. Pat. No. 8,075,603 B2 describes a fastening system consisting of apolyaxial ball and socket joint used in conjunction with a bone screwhaving threads on one end and a spherical connector on the other endoperating as a pivot point about which a connection assembly moves in apolyaxial fashion. A substantially U-shaped connecting assembly has alower receptacle that operates as a socket for housing an upper retainerring and a lower split retainer ring. The socket is receptive to thespherical connector which is inserted through the lower split retainerring causing a momentary displacement thereof which allows for thepositioning of the spherical connector between the upper and lowerretainer rings. A resilient component positioned between the upperretainer ring and the connecting assembly permits relative predeterminedplacement and retention of the spherical connector relative to theconnector assembly.

SUMMARY

The above polyaxial bone anchors allow for inserting a spherical head ofa bone screw into a receiver by pushing the head against a spring forceof a resilient member. However, there is still a need for a couplingassembly and a polyaxial bone anchor with such a coupling assembly thatis improved with regard to several aspects, such as the efficiency andsafety of the coupling.

It is an object of the invention to provide a coupling assembly forcoupling a rod to a bone anchoring element, and a polyaxial bone anchorcomprising such a coupling assembly, that provides a safe connection ofthe bone anchoring element to the coupling assembly with a low insertionforce, while also providing a high retention force, and where only asmall amount of axial travel or displacement is needed to insert thebone anchoring element into the coupling assembly.

The coupling assembly includes a receiving part with an accommodationspace for accommodating the head of the bone anchoring element and aretainer element configured to be positioned at least partially in theaccommodation space. The retainer element includes at least one springportion that is compressible in an axial direction and extends in anaxial direction up to at least a position that is higher than a bottomof a channel of the receiving part that is configured to receive a rod.The spring portion can generate a sufficient spring force thatfacilitates a snap-over of the retainer element on the head of the boneanchoring element when the spring portion is biased. The spring portionmay be realized by, for example, a snake spring that is small-sized in aradial direction, as seen from a central axis of the receiving part, andthat can be, for example, larger in an axial direction.

The retainer element may further have at least one horizontal slit atits bottom end that contributes to a low insertion force requirement forinserting the head into the retainer element. Simultaneously, aretention force that holds the head in the receiving part may be highercompared to the insertion force. Therefore, the bone anchoring elementis effectively prevented from being inadvertently pulled-out from alower opening of the receiving part. In addition, because of a small orreduced insertion path, occurrences of milling under the head orsticking out of the head from bone can be avoided or reduced.

The coupling assembly may further include a pressure element forexerting pressure onto the head of the bone anchoring element to lockthe bone anchoring element in a specific angular position relative tothe receiving part. The retainer element may encompass at least aportion of the pressure element from an outer side thereof, so thatincreasing a height of the receiving part for accommodating both theretainer element and the pressure element may not be necessary. Hence, alow profile implant can be provided.

In addition, the receiving part is monolithic and sized such that theretainer element and the pressure element can be mounted from the topopening thereof.

The pressure element may be held in a position such that the head of thebone anchoring element is held by a frictional force exerted by thepressure element onto the head. The frictional force may be such thatthe head can still be pivoted by applying a force to overcome thefrictional force.

The coupling assembly can be assembled in situ with a bone anchoringelement that has been already inserted into a bone or a vertebra.

With a coupling assembly according to embodiments of the invention, amodular polyaxial bone anchoring device can be provided that may includeseveral bone anchoring elements that differ with respect to the lengthof the shank, anchoring features of the shank, such as different threadtypes and/or thread pitches, different diameters of the shank, and/orwith respect to the shank being cannulated or non-cannulated, amongother features.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become moreapparent from the description of various embodiments using theaccompanying drawings. In the drawings:

FIG. 1 shows a perspective exploded view of a first embodiment of a boneanchoring device.

FIG. 2 shows a perspective view of the bone anchoring device of FIG. 1in an assembled state.

FIG. 3 shows a perspective view from above of a receiving part accordingto the first embodiment.

FIG. 4 shows a perspective view from a bottom of the receiving partshown in FIG. 3.

FIG. 5 shows a top view of the receiving part shown in FIGS. 3 and 4.

FIG. 6 shows a cross-sectional view of the receiving part shown in FIGS.3 to 5, the cross-section being taken along line A-A in FIG. 5.

FIG. 7 shows a perspective view from above of a retainer elementaccording to the first embodiment.

FIG. 8 shows a perspective view from a bottom of the retainer element ofFIG. 7.

FIG. 9 shows a side view of the retainer element of FIGS. 7 and 8.

FIG. 10 shows a top view of the retainer element of FIGS. 7 to 9.

FIG. 11 shows a cross-sectional view of the retainer element of FIGS. 7to 10, the cross-section being taken along line B-B in FIG. 10.

FIG. 12 shows a perspective view from above of a pressure elementaccording to the first embodiment.

FIG. 13 shows a perspective view from a bottom of the pressure elementof FIG. 12.

FIG. 14 shows a top view of the pressure element of FIGS. 12 and 13.

FIG. 15 shows a cross-sectional view of the pressure element of FIGS. 12to 14, the cross-section being taken along line D-D in FIG. 14.

FIG. 16 shows a cross-sectional view of a coupling assembly according tothe first embodiment in an assembled state, the cross-section beingtaken in a plane perpendicular to an axis of a rod to be inserted in thecoupling assembly.

FIG. 17 shows a cross-sectional view of the coupling assembly of FIG.16, rotated by 90°.

FIGS. 18 and 19 show cross-sectional views in planes perpendicular tothe rod axis and rotated by 90°, respectively, of the coupling assemblyduring first step of mounting or inserting the bone anchoring element.

FIGS. 20 and 21 show cross-sectional views in planes perpendicular tothe rod axis and rotated by 90°, respectively, of a second step ofinserting the bone anchoring element into the coupling assembly.

FIG. 22a shows a cross-sectional view in a plane perpendicular to therod axis of a third step of inserting the bone anchoring element intothe coupling assembly.

FIG. 22b shows an enlarged cross-sectional view of a detail of FIG. 22a.

FIG. 23 shows a cross-sectional view of the coupling assembly of FIG.22a , the plane of the cross-section being rotated by 90° from FIG. 22a, i.e., in a plane containing the rod axis.

FIGS. 24 and 25 show cross-sectional views in planes perpendicular tothe rod axis and rotated by 90°, respectively, of a fourth step ofinserting the bone anchoring element into the coupling assembly.

FIGS. 26 and 27 show a cross-sectional view in planes perpendicular tothe rod axis and rotated by 90°, respectively, of a final step ofassembling the coupling assembly and the bone anchoring element.

FIGS. 28 and 29 show cross-sectional views of the fully assembledpolyaxial bone anchoring device according to the first embodiment, withinserted rod and locking element, in a plane perpendicular to the rodaxis and in a plane rotated by 90°, respectively.

FIG. 30 shows a perspective exploded view of a polyaxial bone anchoringdevice according to a second embodiment.

FIG. 31 shows a cross-sectional view of the polyaxial bone anchoringdevice of FIG. 30 in an assembled state, the cross-section being takenin a plane perpendicular to an axis of an inserted rod.

FIG. 32 shows a perspective view from above of a receiving part of thepolyaxial bone anchoring device according to the second embodiment.

FIG. 33 shows a perspective view from a bottom of the receiving part ofFIG. 32.

FIG. 34 shows a bottom view of the receiving part of FIGS. 32 and 33.

FIG. 35 shows a top view of the receiving part of FIGS. 32 to 34.

FIG. 36 shows a cross-sectional view of the receiving part of FIGS. 32to 35, the cross-section being taken along line F-F in FIG. 35.

FIG. 37 shows a perspective view of a retainer element of the polyaxialbone anchoring device according to the second embodiment.

FIG. 38 shows a side view of the retainer element of FIG. 37.

FIG. 39 shows a top view of the retainer element of FIGS. 37 and 38.

FIG. 40 shows a perspective view from above of a pressure element of thepolyaxial bone anchoring device according to the second embodiment.

FIG. 41 shows a perspective view from a bottom of the pressure elementof FIG. 40.

FIG. 42 shows a cross-sectional view of the pressure element of FIGS. 40and 41, the cross-section being taken in a plane perpendicular to anaxis of an inserted rod.

DETAILED DESCRIPTION

As shown in FIGS. 1 and 2, a bone anchoring device according to a firstembodiment includes a bone anchoring element 1 in the form of a bonescrew having a threaded shank 2 and a head 3. The head 3 has a sphericalsegment-shaped outer surface portion 3 a, including a greatest outerdiameter E of the sphere, and a free end with a recess 3 b forengagement with a screwing-in tool. The bone anchoring device furtherincludes a coupling assembly 4 for coupling a stabilization rod 100 tothe bone anchoring element 1. The coupling assembly 4 includes areceiving part 5, and a retainer element 6 and a pressure element 7configured to be arranged in the receiving part 5. The pressure element7 serves for locking the head 3 in the receiving part 5. Pins 8 a, 8 bmay also be provided for holding the pressure element 7 in the receivingpart 5.

In addition, a locking element 9 in the form of an inner screw isprovided for securing the rod 100 in the receiving part 5 and forlocking the whole device.

Referring in particular to FIGS. 3 to 6, the receiving part 5 is amonolithic part that has a first end or top end 5 a and an oppositesecond end or bottom end 5 b, and a central axis of symmetry C passingthrough the top end 5 a and the bottom end 5 b. A bore 51 is providedthat is coaxial with the central axis C. In a first region adjacent tothe first end 5 a, the receiving part 5 has a substantially U-shapedrecess 52 with a bottom directed towards the bottom end 5 b and two freelateral legs 52 a, 52 b extending towards the top end 5 a. On the legs52 a, 52 b, an internal thread 53 is provided that cooperates with thelocking element 9. The channel formed by the U-shaped recess 52 is sizedso as to receive the rod 100 therein for connecting a plurality of boneanchoring devices. In a region below the legs 52 a, 52 b, there is anaccommodation space 54 configured for receiving the head 3 of the boneanchoring element 1 and for receiving at least partially the retainerelement 6 and the pressure element 7. The accommodation space 54 has afirst section 54 a with a slightly larger upper inner diameter than theinner diameter of the coaxial bore 51, and which tapers and narrowstowards the bottom end 5 b of the receiving part 5. In this embodiment,the section 54 a has a hollow cone shape, however, other shapes may alsobe contemplated. The accommodation space 54 further has a second portion54 b that conically tapers towards the bottom end 5 b. The secondportion 54 b forms a seat for a portion of the retainer element 6.Between the two portions 54 a, 54 b, a shoulder 54 c is provided thatprojects inward and serves as a stop for another portion of the retainerelement 6, as described in greater detail below.

The accommodation space 54 further has an opening 55 at the bottom end 5b, the inner diameter of which is larger than the greatest outerdiameter E of the head 3 of the bone anchoring element 1, so that thehead 3 can be inserted into the receiving part 5 from the bottom end 5b. The legs 52 a, 52 b each includes a through-hole in the form of atransverse pin hole 56 a, 56 b that is located substantially centrallyin the legs 52 a, 52 b, and that serves for receiving the pins 8 a, 8 b.The pins 8 a, 8 b have a length such that once a pin 8 a, 8 b isinserted into a respective pin hole 56 a, 56 b, the pin 8 a, 8 b extendsa short distance into the bore 51 to provide a stop for securing arotational position of the pressure element 7. Furthermore, the pins 8a, 8 b also have a function of providing an abutment for a springportion of the retainer element 6. The pins 8 a, 8 b may be flush withan outer surface of the receiving part 5 when inserted in the pin holes56 a, 56 b.

Referring in more detail to FIGS. 7 to 11, the retainer element 6 willbe explained. The retainer element 6 is a substantially sleeve-likepart, with a first end or top end 6 a and an opposite second end orbottom end 6 b. The retainer element 6 is hollow between the top end 6 aand the bottom end 6 b. The retainer element includes a first portion 61in the form of a closed ring that has an outer conical surface taperingand narrowing towards the bottom end 6 b and an inner hollow cylindricalsurface. The first portion 61 is sized so that the first portion 61 fitsinto the lower portion of the first section 54 a of the accommodationspace 54 and abuts against the conical inner surface of the firstsection 54 a. An inner diameter of the inner cylindrical surface of thefirst portion 61 is greater than the largest outer diameter E of thehead 3.

Between the first portion 61 and the bottom end 6 b, the retainerelement 6 has the form of a slit ring 62. The slit ring 62 has asubstantially conical outer surface that matches the inner surface ofthe second section 54 b of the accommodation space 54. An inner surface62 a of the slit ring 62 has the shape of a hollow spherical segmentthat matches the spherical shape of the head 3 so that the slit ring 62provides a seat for the head 3, to form a ball and socket joint betweenthe receiving part 5 and the bone anchoring element 1. The slit ring 62is formed by a first vertical slit 62 b extending from the bottom end 6b in a substantially vertical direction. From the vertical slit 62 b twoopposite horizontal slits 62 c, 62 d extend circumferentially around thecentral axis C. The horizontal slits 62 c, 62 d end in widened endportions 62 e, 62 f The portion between the end portions 62 e, 62 fforms a connecting portion 62 g that connects the slit ring 62 to thefirst portion 61 of the retainer element 6. Hence, the slit ring 62 isintegrally connected to the rest of the retainer element 6. By means ofthe slits 62 b, 62 c, 62 d, the slit ring 62 is configured to beexpanded and compressed in a radial direction. The widths of thevertical slit 62 b and of the horizontal slits 62 c, 62 d as well as thewidth of the connecting portion 62 g, may be selected such that adesired flexibility of the slit ring 62 is obtained.

From the first portion 61, two upstanding rod supporting projections 63a, 63 b that are offset from each other by 180° extend towards the topend 6 a. The upstanding projections 63 a, 63 b each has an outer and aninner cylindrical surface portion, and a groove 64 provided at its freeend that may be substantially V-shaped and sized so as to receive rodsof different diameters. The rod supporting projections 63 a, 63 b extendup to a height, such that a bottom of the V-shaped groove 64 ispositioned slightly above the bottom of the U-shaped recess 52 of thereceiving part 5 when the retainer element 6 is seated in the receivingpart 5. Beneath the bottom of the V-shaped groove 64, a transverse pinhole 65 a, 65 b is respectively provided at each of the rod supportingprojections 63 a, 63 b. The pin holes 65 a, 65 b extend fully throughthe rod supporting projections 63 a, 63 b and serve for receiving a pinfor coupling the retainer element 6 to the pressure element 7, asexplained in greater detail below.

Between the rod supporting projections 63 a, 63 b, there are two springportions 66 a, 66 b, that are respectively provided on either side ofthe V-shaped groove 64 and that project in an axial direction over therod supporting projections 63 a, 63 b, such that the top end 6 a of theretainer element 6 is formed by the end surfaces of the spring portions66 a, 66 b. The spring portions 66 a, 66 b each have an outercylindrical surface with an outer diameter that is only slightly smallerthan an inner diameter of the bore 51 of the receiving part 5 so that,as can be seen for example in FIG. 26, the spring portions 66 a, 66 bfit into the bore 51. The spring portions 66 a, 66 b further have aninner cylindrical surface that is configured to fit around an outercylindrical surface of the pressure element 7, as can also be seen inFIG. 26. The design of the spring portions 66 a, 66 b resembles that ofa snake spring. Each spring portion 66 a, 66 b has alternatingsubstantially vertical sections 67 and horizontal sections 68, whereinthe horizontal sections 68 are preferably longer than the verticalsections 67, and wherein the vertical sections 67 and the horizontalsections 68 form a meandering structure, meandering around a centralaxis c that extends parallel to the central axis C of the receiving part5 when the retainer element 6 is seated in the receiving part 5. Bymeans of this, the retainer element 6 is compressible in a verticaldirection when pressure is exerted onto the spring portions 66 a, 66 b,such that a height of the spring portions 66 a, 66 b change from a firstheight to a second height that is smaller than the first height. Anuppermost horizontal section 68 a of each of the spring portions 66 a,66 b has a free end and may further have a cylinder segment-shapedrecess 69 for receiving the pins 8 a, 8 b. It shall be noted that thelengths of the vertical sections 67 and the horizontal sections 68, andother parameters of the spring portions 66 a, 66 b, may be selected suchthat a desired resiliency is achieved. Also, the vertical sections 67and the horizontal sections 68 need not be exactly vertical orhorizontal, but may instead be inclined. The spring portions 66 a, 66 bmay be easily manufactured by cutting a cylindrical tube. A height ofthe retainer element 6 is such that when the retainer element 6 isseated in the receiving part 5 with the slit ring 62 arranged in thesecond portion 54 b of the accommodation space 54, the uppermosthorizontal sections 68 a of the spring portions 66 a, 66 b abut againstthe pins 8 a, 8 b provided in the receiving part 5.

Referring in greater detail to FIGS. 12 to 15, the pressure element 7 isa substantially cylindrical part, with a first end or top end 7 a and anopposite second end or bottom end 7 b. Adjacent to the bottom end 7 b,there is a substantially spherical segment-shaped recess 71 that matchesthe spherical surface of the head 3 and provides a head contactingsurface portion. Adjacent to the top end 7 a, a rod receiving channel 72is provided that has two substantially flat and parallel side walls 72a, 72 b and a substantially V-shaped bottom 72 c. The size of thechannel is such that a distance between the side walls 72 a, 72 b isonly slightly larger than a diameter of a largest rod to be receivedtherein. The pressure element 7 further has a coaxial bore 73 thatallows access to the recess 3 b in the head 3 of the bone anchoringelement 1 with a tool.

As can be seen in particular in FIGS. 12 and 13, at either side of theV-shaped bottom of the channel 72, an elongate hole 74 a, 74 b (only oneof which is shown in the Figures) extends in an axial direction. Theelongate holes 74 a, 74 b are configured to receive pins 10 a, 10 b (seeFIG. 17) for coupling the retainer element 6 and the pressure element 7together. The pins 10 a, 10 b can move in the elongate holes 74 a, 74 bin an axial direction, where the movement is limited by the upper andlower ends of the elongate holes 74 a, 74 b. At each side wall 72 a, 72b, a U-shaped recess 75 a, 75 b extends from the top end 7 a to adistance therefrom. The U-shaped recesses 75 a, 75 b are open to the topend 7 a and are configured to be engaged by the pins 8 a, 8 b. Thebottoms of the recesses 75 a, 75 b provide stops for the pins 8 a, 8 b,respectively.

The bone anchoring device, as a whole or in part, may be made of abio-compatible material, such as a bio-compatible metal or a metalalloy, for example titanium, stainless steel, a nickel titanium alloy,for example Nitinol, or of one or more bio-compatible plastic materials,such as, for example polyether ether ketone (PEEK), or of abio-compatible ceramic material.

The pressure element 7 and the retainer element 6 may be pre-assembledin such a manner that, as can be seen in particular in FIGS. 16 and 17,the pressure element 7 extends into the retainer element 6, such thatthe V-shaped groove 64 of the retainer element 6 is aligned with thechannel 72 of the pressure element 7. The pins 10 a, 10 b are insertedinto the pin holes 65 a, 65 b of the retainer element 6 and extend intothe elongate holes 74 a, 74 b of the pressure element 7. The pins 10 a,10 b are mounted in a substantially press-fit manner in the holes 65 a,65 b. By means of this, the retainer element 6 and the pressure element7 are movable relative to each other in an axial manner, whereby themovement is limited by the abutment of the pins 10 a, 10 b at the upperand lower ends of the elongate holes 74 a, 74 b of the pressure element7.

The pre-assembled retainer element 6 and pressure element 7 are insertedinto the receiving part 5 from the top end 5 a through the coaxial bore51, so that the slit ring 62 is seated in the second portion 54 b of theaccommodation space 54, as can be seen in FIGS. 16 and 17. As furtherdepicted in FIGS. 16 and 17, after the pre-assembled retainer element 6and pressure element 7 have been inserted into the receiving part 5, thetwo pins 8 a, 8 b are inserted into the pin holes 56 a, 56 b of thereceiving part 5 until their front ends project into the coaxial bore51. A first function of the pins 8 a, 8 b is to provide a stop for thespring portions 66 a, 66 b of the retainer element 6. A second functionof the pins 8 a, 8 b is to limit the path of movement of the pressureelement 7 in the direction towards the top end 5 a of the receiving part5. A third function of the pins 8 a, 8 b is to secure the pressureelement 7 and the retainer element 6 against rotation in the receivingpart 5.

As shown in particular in FIG. 17, during assembly of the retainerelement 6 with the pre-assembled pressure element 7, the pins 8 a, 8 bare located at the upper end of the U-shaped recesses 75 a, 75 b. Theretainer element 6 is pressed down by the two spring portions 66 a, 66b. The pressure element 7 rests with the upper end of the elongatedholes 74 a, 74 b on the pins 10 a, 10 b, respectively, and is stillmovable with respect to the retainer element 6.

The slit ring 62 is seated in the conical second portion 54 b of theaccommodation space 54, and the pins 8 a, 8 b hold the spring portions66 a, 66 b. When the slit ring 62 is positioned in the seat 54 b, thelower edge 6 b of the retainer element 6 projects slightly out of thelower opening 55 receiving part 5.

Next, as depicted in FIGS. 18 and 19, the head 3 of the bone anchoringelement 1 is inserted into the receiving part 5 through the loweropening 55. The head 3 may first begin entering the lower portion of theslit ring 62 that projects slightly through or out of the lower opening55.

The slit ring 62 is pushed upwards out of its seat 54 b by the head 3,as further depicted in FIGS. 20 and 21. The assembly of the retainerelement 6 and the pressure element 7 is moved upward towards the top end5 a of the receiving part 5, whereby the pins 8 a, 8 b enter deeper intothe V-shaped recesses 75 a, 75 b of the pressure element 7. During theupward movement, the spring portions 66 a, 66 b are compressed, becausetheir upper ends 68 a with the recesses 69 abut against the pins 8 a, 8b. In addition, the slit ring 62 is expanded when the head 3 entersfurther into it. Because the retainer element 6 and the pressure element7 are coupled together via the pins 10 a, 10 b, the pressure element 7may also be moved upwards.

Referring further to FIGS. 22a to 23, further insertion of the head 3causes further expansion of the slit ring 62 and further compression ofthe spring portions 66 a, 66 b. When a counterforce exerted by thecompressed spring portions 66 a, 66 b is greater than a force needed forexpanding the slit ring 62 and for sliding the slit ring 62 over theportion of the head 3 having the largest diameter E, the spring force ofthe compressed spring portions 66 a, 66 b causes the slit ring 62 tosnap over the head 3 so that the lower edge 6 b of the retainer element6 slides over the region of the head 3 with the largest diameter E (seeFIG. 22b ). The head 3 can be inserted into the coupling assembly 4 onlyto such an extent that the head 3 abuts against the spherical recess 71of the pressure element 7. The bottoms of the U-shaped recesses 75 a, 75b of the pressure element 7 serve as abutments for the pins 8 a, 8 b, sothat the pressure element 7 and the retainer element 6 cannot escapethrough the top end 5 a of the receiving part 5.

As shown in FIGS. 24 and 25, after the head 3 is fully inserted in thecoupling assembly 4, where the pins 8 a, 8 b abut against the bottom ofthe U-shaped recesses 75 a, 75 b, the spring portions 66 a, 66 b of theretainer element 6 may expand, thereby moving the pins 10 a, 10 bdownward until they abut against the bottom of the elongate holes 74 a,74 b of the pressure element 7. The slit ring 62 is positioned below theportion of the head 3 with the largest diameter E. The sliding of theretainer element 6 over the head 3 is supported or facilitated by thespring portions 66 a, 66 b, and is therefore quick and safe. When theslit ring 62 is below the portion of the head 3 with the greatestdiameter E, the head 3 can no longer be pulled-out through the loweropenings of either the receiving part 5 or the retainer element 6.

As further shown in FIGS. 26 and 27, when the receiving part 5 is pulledupward or the pressure element 7 is pressed downward, the head 3 pressesor urges the slit ring 62 into the seat 54 b. This condition is apre-locking condition, wherein the head 3 is connected to the couplingassembly 4, such that the head 3 cannot be separated from the couplingassembly 4 without applying or utilizing a special instrument.

The inner portion of the slit ring 62 may have a slight undersize withrespect to the size of the head 3, such that the slit ring 62 exerts africtional force on the head 3 when the slit ring 62 is around the head3. Hence, in the condition shown in FIGS. 26 and 27, the head 3 is heldby friction within the receiving part 5, and therefore, the receivingpart 5 can be maintained at a specific angular position with respect tothe bone anchoring element 1 before locking of the bone anchoringelement 1 relative to the receiving part 5. Also the spring forceexerted by the spring portions 66 a, 66 b may further contribute to thefriction hold of the head 3 in the receiving part 5 by urging the slitring 62 into the seat 54 b.

When the rod 100 is mounted in the receiving part 5 and moved downwardwith the locking element 9, the pressure element 7 presses against thehead 3, and the head 3 presses against the slit ring 62. Finaltightening of the locking element 9 locks the whole device. The fullylocked condition is shown in FIGS. 28 and 29.

In use, the bone anchoring element 1 may be inserted into a bone or intoa vertebra prior to mounting the coupling assembly 4. In an alternativemanner of use, the bone anchoring element 1 and the coupling assembly 4are pre-assembled and thereafter can be inserted into the bone orvertebra. A plurality of bone anchoring devices can be connected throughthe stabilization rod 100.

It shall be noted that while two pairs of pins 8 a, 8 b and 10 a, 10 bare shown, the basic functions of the pins may be achieved also withonly one pin for coupling the retainer element 6 and the receiving part7 together and only one pin for forming an abutment for the springportions 66 a, 66 b and for holding the pressure element 7 and theretainer element 6 in the receiving part 5.

Referring to FIGS. 30 and 31, a second embodiment of the polyaxial boneanchoring device will be described. The polyaxial bone anchoring deviceaccording to the second embodiment differs from the polyaxial boneanchoring device according to the first embodiment in the design of thecoupling assembly. All parts that are similar or identical to theprevious embodiment are marked with the same reference numerals, and thedescriptions thereof will not be repeated.

The coupling assembly 4′ includes a receiving part 5′, a retainerelement 6′, a pressure element 7′, and one pin 8′. Referring in additionto FIGS. 32 to 36, the receiving part 5′ has an asymmetric accommodationspace 54′. The accommodation space 54′ has a first portion 54 a′ thatextends between the coaxial bore 51 and a seat portion 54 b′ that has aspherical segment shape with a limited length in a circumferentialdirection. The size of the spherical section of the seat portion 54 b′matches the size of a lower portion of the head 3. Hence, the sphericalsection 54 b′ provides a partial spherical seat for the head 3. From thespherical section 54 b′, a compartment 54 d′ extends from a bottomshoulder 54 c′ up to a height that is above (i.e., closer to the top end5 a than) a bottom of the U-shaped recess 52. The compartment 54 d′enlarges the accommodation space asymmetrically with respect to thecentral axis C, so that at one of the legs 52 a, there is an enlargedspace for receiving the retainer element 6′ according to the secondembodiment. The lower edge 54 c′ of the compartment 54 d′ serves as asupport for supporting the retainer element 6′, as shown in FIG. 31. Theaccommodation space 54′ of the receiving part 5′ has a greater diameterat a region of the compartment 54 d′ than at a region of the sphericalsection 54 b′. In addition, a transverse pin hole 56′ for receiving thepin 8′ and a circumferential extending undercut portion 57′ above thepinhole 56′ are provided in the receiving part 5′.

The retainer element 6′ includes a main portion 61′ that has an outercylinder segment-shaped contour 61 a and an inner sphericalsegment-shaped contour 61 b, which together resemble a sickle or acrescent shape from a top view (for example, as seen in FIG. 39). Themain portion 61′ of the retainer element 6′ has a height and a width ina circumferential direction that allows the main portion 61′ toencompass at least a portion of the spherical head 3. The main portion61′ fits into a lower portion of the compartment 54 d′, such that themain portion 61′ can rest on the support 54 c′. From the main portion61′, a spring portion 66′ extends upward towards the top end 6 a. Thespring portion 66′ is shaped similar to a snake spring, and has asickle-shaped or crescent-shaped contour from a top view, as shown inFIG. 39. A number and the lengths of, as well as the distances between,the substantially vertical sections 67′ and the substantially horizontalsections 68′ that form the meandering design of the spring portion 66′can be adjusted or selected to achieve a desired resiliency and springforce. The retainer element 6′ is configured to be inserted into thecompartment 54 d′. Once the retainer element 6′ is inserted into thecompartment 54 d′, the retainer element 6′ extends in an oblique ortilted manner relative to the central axis C of the coupling assembly4′, as depicted in FIG. 31.

The pressure element 7′ includes at a top end 7 a two lateral collarportions 76 a, 76 b that extend from a V-shaped groove 72 c beyond anouter diameter of a cylindrical body of the pressure element 7′. Thecollar portions 76 a, 76 b have an outer size such that the collarportions 76 a, 76 b can extend into the undercut portion 57′ providedabove the pin hole 56′ in the receiving part 5′. Below one of the collarportions, e.g., the collar portion 76 b in the embodiment shown, anelongate U-shaped recess 75′ is provided that is open towards a bottomend 7 b of the pressure element 7′. The U-shaped recess 75′ serves forreceiving a portion of the pin 8′ therein.

Referring in particular to FIG. 31, the coupling assembly may bepre-assembled such that the retainer element 6′ is inserted into thereceiving part 5′ from the top end 5 a and through the coaxial bore 51,until the retainer element 6′ is seated in the compartment 54 d′, andthe main portion 61′ rests on the support 54 c′. The height of thecompartment 54 d′ is such that the spring portion 66′ may be slightlypre-tensioned when the retainer element 6′ is placed into thecompartment 54 d′. The inner spherical surface 61 b extends into theaccommodation space 54′ and forms a second portion of a seat for thehead 3, that encompasses a portion of the head 3 on a side opposite aside of the spherical seat 54 b′.

The pressure element 7′ is also inserted from the top end 5 a in amanner such that the collar portions 76 a, 76 b are aligned with the rodreceiving channel 52 of the receiving part 5′. Then the pressure element7′ is moved downward until the collar portions 76 a, 76 b reach theundercut portion 57′. In this position, the pressure element 7′ can berotated so that the V-shaped groove 72 c is aligned with the channel 52of the receiving part 5′, and the collar portions 76 a, 76 b stillextend into the undercut portion 57′. Thereafter, the pin 8′ is insertedthrough the pin hole 56′ until its front portion extends into theU-shaped recess 75′ of the pressure element 7′. The pin 8′ preventsrotation of the aligned pressure element 7′.

In use, the head 3 enters through the lower opening 55′ and moves theretainer element 6′ upward, thereby compressing the spring 66′ thatmoves or pushes against the upper wall of the oblique compartment 54 d′.The pressure element 7′ is also pushed upward by the head 3 until itsfree end surface 7 a, provided on the collar portions 76 a, 76 b, abutsagainst an upper surface of the undercut 57′. When the head 3 pressesagainst the spherical recess 71 of the pressure element 7′, the springforce of the spring portion 66′ causes the retainer element 6′ to snapover or past the portion of the head 3 with the largest diameter E untilthe main portion 61′ of the retainer element 6′ rests on the support 54c′ in the receiving part 5′. In this position, the head 3 can no longerbe removed from the receiving part 5′.

Further modifications of the embodiments may also be contemplated. Forexample, for the bone anchoring element, various different kinds ofanchoring elements can be used and combined with the receiving parts.Such anchoring elements may be, for example, screws with differentlengths, screws with different diameters, cannulated screws, screws withdifferent thread forms, nails, hooks, etc. For some anchoring elements,the head and the shank may also be separate parts that can be connectedto each other.

Other possible modifications of the receiving part may include, forexample, instead of having a U-shaped recess being perpendicular to thecentral axis, a recess for the rod may be inclined, open to the side ofthe receiving part, or may be realized in the form of a closed channel.Other kinds of locking devices, including outer nuts, outer caps,bayonet locking devices, or others are also possible. In particular, atwo-part locking device that includes a first locking element thatexerts pressure via the pressure member only onto the head and a secondlocking element that exerts pressure only onto the rod to lock the headand the rod independently, may also be used. In some embodiments, theinner surface portion of the pressure member that contacts the head, maynot necessarily be spherically-shaped. The inner surface portion mayinstead have any other shape that is suitable to exert pressure onto thehead.

In some embodiments, instead of a pin that extends through a pin hole atthe receiving part and engages an elongate recess provided at thepressure element, other retaining mechanisms can be used that helpretain the pressure member in alignment with the receiving part andinhibit or restrict the pressure member from moving out through the topend of the receiving part.

The head of the bone anchoring element need not be rotationallysymmetric. For example, the head may have two opposite flat surfaceportions between two spherically-shaped outer surface portions, so as toachieve pivoting in only one plane.

The spring portion or portions of the retainer element may have othershapes that ensure sufficient length of the spring portion in an axialdirection and sufficient spring force. For example, a helical spring mayalso be contemplated.

Instead of a slit ring, a plurality of vertically extending slits or acombination of substantially vertically and substantially horizontallyextending slits may instead be provided.

The seat for the slit ring and the outer surface of the slit ring alsoneed not be conical. Any shape that provides for safe holding of theslit ring in the receiving part may also be contemplated, such as, forexample, a spherical shape.

While the present invention has been described in connection withcertain exemplary embodiments, it is to be understood that the inventionis not limited to the disclosed embodiments, but is instead intended tocover various modifications and equivalent arrangements included withinthe spirit and scope of the appended claims, and equivalents thereof.

1. A coupling assembly for coupling a rod to a bone anchoring element,the coupling assembly comprising: a receiving part having a first end, asecond end, a central axis extending through the first end and thesecond end, a recess at the first end for receiving the rod, the recessextending axially from the first end down to a bottom of the recess, anaccommodation space for accommodating a head of the bone anchoringelement, and an opening at the second end of the receiving part toprovide access to the accommodation space; a monolithic retainer elementcomprising a head contacting portion and a spring portion, wherein whenthe retainer element is in the receiving part, the head contactingportion is configured to extend into the accommodation space to directlycontact the head, while the spring portion is compressible in adirection parallel to the central axis and is configured to extend in anaxial direction to a position above the bottom of the recess; and apressure element that is separable from the retainer element and thathas at least a portion positionable in the accommodation space to exertpressure on the head.
 2. The coupling assembly of claim 1, wherein thespring portion comprises a section that extends in a meandering manneralong a longitudinal axis that is substantially parallel to or slightlyinclined with respect to the central axis of the receiving part when theretainer element is in the receiving part.
 3. The coupling assembly ofclaim 1, wherein an outer contour and an inner contour of the springportion is substantially cylinder segment-shaped.
 4. (canceled)
 5. Thecoupling assembly of claim 1, wherein the spring portion extends towardsthe first end of the receiving part to an axial position correspondingto a center of an inserted rod.
 6. The coupling assembly of claim 1,wherein the pressure element is positionable at least partially in theaccommodation space to exert pressure directly on the head.
 7. Thecoupling assembly of claim 1, wherein the spring portion of the retainerelement extends at least partially around the pressure element in acircumferential direction when the retainer element and the pressureelement are in the receiving part.
 8. The coupling assembly of claim 1,wherein a bore extends from the first end of the receiving part to theaccommodation space and is sized such that the pressure element isinsertable into the receiving part from the first end.
 9. The couplingassembly of claim 1, wherein a stop is provided for the pressure elementto restrict movement of the pressure element towards the first end ofthe receiving part when the pressure element is in the receiving part.10. (canceled)
 11. The coupling assembly of claim 1, wherein when theretainer element and the pressure element are in the receiving part, theretainer element and the pressure element are configured to be coupledtogether and to remain axially movable relative to each other over alimited distance.
 12. The coupling assembly of claim 1, wherein theretainer element has a first end, a second end, a first slit spacedapart from the second end that extends at least partially around alongitudinal axis of the retainer element, and a second slit thatextends from the second end of the retainer element to the first slit,and wherein the first slit extends away from the second slit and islonger than the second slit.
 13. The coupling assembly of claim 12,wherein the retainer element comprises a third slit extending partiallyaround the longitudinal axis in a direction away from the first slit andthe second slit, such that a slit ring is formed at the second end ofthe retainer element.
 14. The coupling assembly of claim 1, wherein thespring portion of the retainer element comprises two separate springportions configured to be positioned on either side of the recess of thereceiving part when the retainer element is in the receiving part. 15.The coupling assembly of claim 1, wherein when the retainer element isin the receiving part, the spring portion extends in an oblique ortilted manner relative to the central axis of the receiving part withinan enlarged compartment of the receiving part, and wherein the enlargedcompartment provides an upper stop and a lower stop for the retainerelement.
 16. A polyaxial bone anchoring device for coupling a rod to abone, the polyaxial bone anchoring device comprising: a bone anchoringelement comprising a shank for anchoring to bone and a head; and acoupling assembly comprising: a receiving part having a first end, asecond end, a central axis extending through the first end and thesecond end, a recess at the first end for receiving the rod, the recessextending axially from the first end down to a bottom of the recess, anaccommodation space for accommodating a head of the bone anchoringelement, and an opening at the second end of the receiving part toprovide access to the accommodation space; a retainer element comprisinga spring portion; and a pressure element separable from the retainerelement; wherein when the head, the retainer element, and the pressureelement are in the receiving part, the spring portion is compressible ina direction parallel to the central axis, and the retainer element andthe pressure element are both movable axially in the receiving part andboth configured to directly contact the head while also being configuredto extend axially to respective positions above the bottom of therecess.
 17. The polyaxial bone anchoring device of claim 16, wherein theretainer element is configured to at least partially encompass the head.18. The polyaxial bone anchoring device of claim 16, wherein the springportion is monolithically formed with other parts of the retainerelement.
 19. The polyaxial bone anchoring device of claim 16, whereinwhen the head, the retainer element, and the pressure element are in thereceiving part, the bone anchoring device is configured to assume alocking position where an angular position of the bone anchoring elementrelative to the receiving part is locked while the spring portionextends to a position above the bottom of the recess.
 20. The couplingassembly of claim 9, wherein the stop also restricts movement of theretainer element towards the first end of the receiving part when theretainer element is in the receiving part.
 21. The coupling assembly ofclaim 1, wherein when the retainer element and the pressure element arein the receiving part, at least part of the retainer element isconfigured to extend down to an axial position that is closer to thesecond end of the receiving part than the entire pressure element is tothe second end of the receiving part.
 22. A polyaxial bone anchoringdevice for coupling a rod to a bone, the polyaxial bone anchoring devicecomprising: a bone anchoring element comprising a shank for anchoring tobone and a head; and a coupling assembly comprising: a receiving parthaving a first end, a second end, a central axis extending through thefirst end and the second end, a recess at the first end for receivingthe rod, the recess extending axially from the first end down to abottom of the recess, an accommodation space for accommodating a head ofthe bone anchoring element, and an opening at the second end of thereceiving part to provide access to the accommodation space; and aretainer element comprising a monolithic spring portion; wherein whenthe head and the retainer element are in the receiving part, the springportion is elastically compressible in a direction parallel to thecentral axis, and the retainer element is movable to a locking positionwhere an angular position of the bone anchoring element relative to thereceiving part is locked while the spring portion extends to a positionabove the bottom of the recess.